Chicken fetal antigen (CFA) disappears from the cell membrane with chicken development and reappears on the cellular membrane of adult chickens with AMV-induced leukemia. The fetal-normal adult-leukemic differences as to CFA are found on several cell types, including the red blood cells. The major objective is to determine the genetic mechanism(s) coding for the disappearance of CFA with development and the subsequent reappearance of CFA in adult leukemic chickens. Two hypotheses have been advanced to interpret the data, namely, fetal gene derepression, and blockage in cell differentiation. Cellular and intracellular markers are being utilized in order to discern between the above two hypotheses. Somatic hybridization studies between embryonic Japanese quail red blood cells and adult chicken red blood cells are being conducted in order to determine the genetic regulation of CFA in cell hybrids. Ferratin labeled anti-CFA will be utilized in order to follow the progressive loss of CFA with development, localize CFA to specific cell types, and to localize CFA to specific regions of the cells. Bibliographic references: Sanders, B.G., R.L. Teplitz, A.M. Brodetsky, H. Fung, and K.L. Wiley, 1975. Investigation of fetal antigen in AMV-induced leukemia. Thirteenth Hanford Biology Symposium on the Cell Cycle in Malignancy and Immunity. ERDA publication. pp. 348-358. Dietert, R. and B.G. Sanders, 1975. Somtic cell hybridization and the regulation of avian leukemic antigens. Manuscript in preparation.